The present invention relates to a process for preparing a carrier used in olefin polymerization catalysts, and more particularly, to a process for preparing a carrier used in Ziegler-Natta high-efficiency solid catalyst component.
It is well known that high-efficiency supported catalysts for polymerizing olefins are prepared by using inorganic materials such as magnesium chloride and the like as the carrier and supporting a transition metal compound onto it, and that the properties of the carrier are very important. In early stages, magnesium chloride carriers were mainly prepared by employing a grinding method, which is disadvantageous in that the resultant catalysts have a poor particle morphology and thus the polymers produced by employing such catalysts also have a poor particle morphology, more fine powders and a low apparent density.
Later on, it is proposed to prepare a carrier by employing a chemical method wherein a magnesium compound is dissolved certain solvents and is then precipitated to obtain the carrier having a uniform particle size distribution. For example, Chinese Patent Application Publication No. CN1085569A discloses a process for preparing a titanium catalyst, wherein a magnesium halide is contacted with an alcohol having at least 6 carbon atoms in a hydrocarbon solvent to form a magnesium solution, and is then reacted with an organoaluminium compound to form a solid magnesium-aluminium complex. The solid magnesium-aluminium compolex is suspended in a hydrocarbon solvent and then to the resulting suspension is added a tetravalent titanium compound to form a solid titanium catalyst suitable for polymerizing ethylene.
Now many attempts have been made to further develop supported catalysts having a controllable morphology. By employing such catalysts, it is possible to prepare polymers having a particle morphology similar to that of the catalysts and having high flowability, thus simplifying the procedures involving the preparation and/or post-treatment of the resulting polymers.
U.S. Pat. No. 4,399,054 discloses solid catalyst components for the polymerization of propylene, having a particle size of 1 to 100 microns. The carrier used for the components is an adduct having the formula MgX2.nED, wherein X is a halogen, n is a number from 1 to 3 and ED is an electron-donor compound containing active hydrogen atoms such as ethanol and the like. Said carrier is prepared by passing a homogeneous emulsion of the molten adduct through a pipe under turbulent conditions and quenching the emulsion to partially dealcoholize.
Chinese Patent Application Publication Nos. CN1089622A and CN1047302A also empoly an adduct of a magnesium halide with an alcohol as the carrier for solid catalyst components. The adduct is prepared by adding an alcohol to a magnesium halide in the presence of an inert hydrocarbon solvent to form a solution and has a higher content of alcohol, for example larger than 3 moles, and then the carrier for catalyst components is obtained by removing some of the alcohol from the adduct. The alcohol is removed usually by heating or chemical treatment, and the removal of alcohol by chemical treatment is conducted by adding sufficient amounts of a dealcoholizing agent to react with OH groups contained in the adduct. The dealcoholizing agent is usually selected from consisting of the group of alkyl aluminium such as triethyl aluminium and triisobutyl aluminium, silicon halides and tin halides.
The object of the present invention is to provide a process for preparing a carrier used in olefin polymerization catalysts, which is simple and by which the resultant carrier has a narrow particle size distribution, while the solid catalyst components prepared from the carrier have high catalytic activity.
The present invention in its one aspect provides a process for preparing a carrier used in olefin polymerization catalysts, comprising suspending anhydrous magnesium chloride in an inert hydrocarbon solvent and then under stirring, activating the magnesium chloride with a C2-C8 alcohol at a temperature of 30xc2x0 C. to 200xc2x0 C., preferably 60xc2x0 C. to 150xc2x0 C., for 0.3 to 5.0 hours, with the molar ratio of said alcohol to said magnesium chloride being in the range of 0.05 to 2.5, preferably 0.1 to 1.0.
The present invention in its another aspect provides a process for preparing a carrier used in olefin polymerization catalysts, comprising suspending anhydrous magnesium chloride in an inert hydrocarbon solvent, pre-dispersing the magnesium chloride with a dispersing agent, with the molar ratio of said dispersing agent to said magnesium chloride is 0.01 to 2.0, and then under stirring, activating the magnesium chloride with a C2-C8 alcohol at a temperature of 30xc2x0 C. to 200xc2x0 C., preferably 60xc2x0 C. to 150xc2x0 C., for 0.3 to 5.0 hours, with the molar ratio of said alcohol to said magnesium chloride being in the range of 0.05 to 2.5, preferably 0.1 to 1.0.
The C2-C8 alcohol used in the processes of the present invention can be ethanol, propanol, isopropanol, butanol, hexanol, octanol or the mixtures thereof, more preferably ethanol, isopropanol or butanol.
The inert hydrocarbon solvent used in the processes of the present invention can be C5-C15 alkanes or C6-C8 aromatic hydrocarbons, preferably C5-C12 alkanes, more preferably hexane, decane, heptane or octane. The weight ratio of the inert hydrocarbon solvent to anhydrous magnesium chloride is from 5 to 200, preferably from 20 to 100.
Upon activation, the stirring rate is controlled to be 30 to 3000 rpm, preferably 30 to 1000 rpm, most preferably 30 to 600 rpm.
The amount of C2-C8 alcohol used for activating the magnesium chloride in the processes of the present invention should be strictly controlled so that the molar ratio of alcohol/magnesium is in the range of 0.05 to 2.5. When the molar ratio of alcohol/magnesium is larger than 2.5, the magnesium chloride suspension will become too viscous and thus the carrier having a good particle morphology and a uniform particle size distribution can not be obtained; and when the molar ratio of alcohol/magnesium is less than 0.05, the resultant solid catalyst component will have very low activity.
In addition, the particle morphology and particle size distribution of the carrier produced by the processes of the present invention are closely related to the types and amounts of the alcohol used for activating. When ethanol is used for activating, the magnesium chloride suspension will become too viscous in the case of the molar ratio of alcohol/magnesium larger than 2.0 and thus a carrier having a uniform particle size distribution can not be obtained; and when butanol is used for activating, the magnesium chloride suspension will become too viscous in the case of the molar ratio of alcohol/magnesium larger than 1.0 and thus a carrier having a uniform particle size distribution can not be obtained.
Prior to activation by C2-C8 alcohol, anhydrous magnesium chloride may be preferably pre-dispersed by a dispersing agent, so as to avoid the adherence of the carrier during the preparation (which influences the particle morphology and the particle size distribution of the resultant carrier) and to obtain solid catalyst components having higher catalytic activity from the carrier. The conditions for pre-dispersing, such as temperature and the stirring rate, can be same as those in the activation step, and the molar ratio of the dispersing agent to anhydrous magnesium chloride is controlled to be in the range of 0.01 to 2.0. When the molar ratio of the dispersing agent to the magnesium chloride is larger than 2.0, the magnesium chloride will become dissolved or a colloid will be formed, which influences the particle morphology and the particle size distribution of the resultant carrier. The dispersing agent used for pre-dispersing can be selected from the group consisting of alkoxides of titanium having the formula Ti(OR)4, wherein R is C2-C6 alkyl group, and C3-C8 alcohols, preferably tetrabutoxy titanium, tetraethoxy titanium, tetrapropoxy titanium, propanol or butanol.
The carrier produced by the processes of the present invention is suitable for preparing solid catalyst components for Ziegler-Natta catalysts and the solid components can be prepared by conventional titanium-supporting methods. For example, the carrier produced by the processes of the present invention is suspended in an inert hydrocarbon solvent, to the suspension is added a titanium halide in an amount such that the molar ratio of magnesium/titanium is 1:1-30, preferably 1:1-10, the resulting mixture is then reacted at a temperature of 30xc2x0 C. to 200xc2x0 C., preferably 60xc2x0 C. to 150xc2x0 C., for 0.5 to 5.0 hours. At the end of the reaction, the resulting solid is washed with an inert hydrocarbon to remove free titanium halide, thereby giving solid catalyst component particles having a particle size of 5 to 200 microns. The titanium halide used can be preferably TiCl4, and the resulting solid catalyst component has a titanium content of 0.2 to 20.0 percent by weight.
The solid catalyst components prepared from the carrier produced by the processes of the present invention are suitable for polymerizing ethylene and copolymerizing ethylene with alpha-olefins. When used for (co)polymerizing, it is necessary to add as cocatalyst an alkyl aluminium, preferably triisobutyl aluminium, triethyl aluminium or tri-n-butyl aluminium, with Al/Ti ratio being suitably 20 to 800, preferably 20 to 300, by mole.
Since in the present invention, the carrier is directly prepared by suspending anhydrous magnesium chloride in an inert hydrocarbon solvent, the process according to the present invention can not only reduce the amount of alcohol used, but also eliminate the dealcoholization step and thus can simplify the preparation of the carrier, in comparison with the conventional processes which comprise firstly preparing a magnesium chloride . alcohol adduct and then partially dealcoholizing. Moreover, the solid catalyst component particles prepared from the carrier produced by the processes of the present invention have high flowability, narrower particle size distribution and higher catalytic activity.